N/A values after running the model

Dear C-PORT Developers,

Thank you so much for developing C-PORT. I find it fascinating.

I’m using C-PORT model to see an impact of a port policy on the air quality and would like to ask some questions. Is it normal to have so many missing values? (If you see the attached image file, about 70% of Total_Concentration value is 0.) Does null value mean there is a model/data constraint or I’ve done some technical mistake or imply there is simply no association from the port terminals?

Thank you so much!